Writer Cavan Scott told us in an interview that this was a natural point to tell new stories about the Doctor because there is an obvious gap between episodes The Doctor Dances and Boom Town. Presumably, though the timing on the TARDIS often leaves much to be desired, every time the TARDIS takes off with only the Doctor in it could be an alternate timeline. Still, it’s a solid choice to set the story here with the TARDIS ‘dream team’ of Nine, Rose and Jack Harkness.